As a method for measuring water content by means of Karl Fischer titration method, there are a volumetric titration method and a coulometric titration method, and there are apparatus corresponding to each method.
The volumetric titration apparatus has a basic configuration as shown in FIG. 8, wherein a titration flask 1 is provided with an injection nozzle 90 for titrating Karl Fischer reagent and an detection electrode 80 for detecting a polarization state. In such configuration, a dehydrated solvent is put in the titration flask 1, which is dehydrated by performing a preliminary titration with the Karl Fischer reagent. In this condition, a sample is put in the titration flask 1, and then drops of Karl Fisher reagent are added therein. According such process, the water contained in the sample reacts with iodine in the reagent. Since a concentration of iodine increases and a voltage detected by the detection electrode 80 falls as the reagent is added therein, a point when the detected voltage becomes a specific value is set to an end point. In this case, since the added amount of the reagent reacts quantitatively with the water content, the water content in the sample can be estimated by the added amount of the reagent.
In order to establish the above-mentioned theory, since it is based on the assumption that the water amount reacting with a specific amount (e.g. 1 ml) of Karl Fischer reagent (which is referred to titer) is known in advance, the titer of the reagent to be used is necessary to be measured previously. This measurement uses a liquid to be standard, such as a standard solution. That is, the standard solution (water produced as standard solution, sodium tartrate dihydrate, and pure water) is injected in the titration flask 1 holding the dehydrated solvent, and Karl Fischer reagent is added therein, so that the water amount reacting with the standard solution is defined as the titer.
In the above volumetric titration, the used Karl Fischer reagent is disposed as matters now standard, because it is no further use for the titer determination as described later.
In the coulometric titration apparatus, the titration flask 1 is provided with an electrolytic electrode 95 for electrolytic treatment of anolyte, and the detection electrode 80 for detecting the polarization state, as shown in FIG. 9.
In the configuration, the anolyte (solution including iodine ions) is put in the titration flask 1 and catholyte is put in a chamber of the electrolytic electrode 95. Since the anolyte absorbs humidity in the air at this time, the anolyte is dehydrated by the electrolytic treatment for generating the iodine at the electrolytic electrode 95 (the preliminary titration). In this condition, the sample is put in the anolyte, and the electrolytic treatment is carried out at the electrolytic electrode 95 to produce the iodine from the iodine ions, and make the iodine react with the water in the sample, (the titration for producing the iodine from the iodine ions is referred to the coulometric titration, hereinafter).
As the coulometric titration proceeds, the concentration of iodine in the titration flask 1 increases and the voltage detected by the detection electrode 80 reduces, therefore a point when the detected voltage becomes a specific value is set to an end point. The water content can be estimated based on the quantity of electricity consumed at this time.
The coulometric titration used here is an electric current control method for controlling voltage so as to fix the current to a constant, and the results by means of the current control method are well-matched with theoretical values, as long as the iodine is produced from the iodine ions.
In the above-mentioned volumetric titration apparatus, a back titration as described hereinafter can be carried out.
Specifically, the iodine in Karl Fischer reagent reacts with the water in the sample as described above. A specific amount of Karl Fischer reagent (water equivalent x) is added in the sample. When the reaction of the water in the sample and the iodine is terminated, the iodine that did not react with the water in the sample has remains, so that the detection electrode 80 indicates a lower value than the end point. Accordingly, when a standard water-methanol is titrated in this condition, the standard water-methanol reacts with the excessive iodine and the iodine ions are produced. When the standard water-methanol is added up to that the voltage indicated by the detection electrode 80 becomes the end point, the water content in the sample can be calculated by x−y, based on the added amount of the standard water-methanol (water equivalent y) and the amount of the Karl Fischer reagent (water equivalent x).
The titration for producing the iodine ion from the iodine in this case, however, is limited to the volumetric titration method. This kind of titration cannot be carried out in the coulometric titration apparatus due to the reasons described later, and the coulometric titration apparatus cannot be applied to the back titration. Moreover, since the coulometric titration apparatus is not provided with a burette for adding the standard water-methanol and the injection nozzle 90, it cannot carry out the back titration functionally.
The Karl Fischer titration apparatus that is incorporated with both the volumetric titration device and the coulometric titration device is put on the market. In this case, operation units such as a display, a keyboard, and the like are shared with both device, but measurement units (the injection nozzle+detection electrode, or electrolytic electrode+detection electrode: but the detection electrode can be shared) are constituted exclusively. Such device, however, is configured so that the independent two methods are merely carried out by the shared operation units, and it is not provided with a unique function like the present invention as described hereinafter.
Prior art citations are Japanese Unexamined Patent Application Publication No. 06-308113 (Patent Literature 1) that discloses a combined type of titration apparatus for carrying out the volumetric analysis or the coulometric analysis, and Japanese Unexamined Patent Application Publication No. 2007-278919 (Patent Literature 2) that discloses a moisture meter for measuring the water content in the sample by the volumetric titration method or the coulometric titration method.